Breathing Air

Aquatic animals get their oxygen from the water, but
land animals have to get it directly from the air. Water evaporates
in air, so land animals risk being dehydrated when they breathe.

Breathing

Multicellular organisms are eucaryotic
and require oxygen for respiration. In the water, animals absorb
oxygen from the water as the water flows over their gills. This
method requires that the gills stay moist, which is not a problem
in water. However, in air the surfaces of the gills dry out,
oxygen absorption stops, and fish die. Clearly a new strategy
is needed in air.

Lungs inside the body provide surfaces
which can be kept moist by fluids in the blood. Lungs are delicate
and complex organs which were developed early in the story of
life (probably by a relative of the modern lungfish) and modified
as necessary as the vertebrate life forms diversified.

The lungs are enclosed in the body,
which helps to keep them from drying out. They are protected
from wind and sudden temperature changes. The lungs are rich
in blood vessels so that they can absorb as much oxygen as possible.

The lungs also give up carbon dioxide.
Carbon dioxide builds up in the body as a waste product of metabolism.
If there is too much of it in the body, the organism will die.

Insects use different structures, called
book lungs to do the same kind of work for their bodies..